Ok, today it happened again and of course it was raining. Luckily I had my 20 mile drive from work to analyze what MAY be happening.

As I was driving on the freeway I wouldn't dare go above 40 mph. The rear end seemed liked I was driving on skis. The rear tires have plenty of thread but you would of thought I was driving in snow how much the rear end seemed to slide to the side.

When there was a line in the road that was parallel to the direction I was driving and my tires hit it, the car would shoot over.

The dead bumping sound didn't start until about 10 miles into the ride. And it would come and go.mwhen it was happening it sounded like I was riding on flat tires.
I think I know what was happening, let me know if this sounds plausible:

I believ it's the rear shocks. It seems as though when the rear shocks get wet they loose their ability to dampen the bumps from the road. I'm assuming the water/moisture breaks the seal long enough for the shocks to just slam up and down.
This would come and go during the last 10 miles. Sometimes it was perfect and others bam bam.

Although the ski like driving continued no matter what. Seems as though the rear alignment is so out of wack that the rear end slides into the direction of the angle they are misaligned.

Yesterday I did check all of the bushings in the rear and visually everything looked good. The rear driver side air bag was installed in May of 2012 and the plastic box with the plastic arm that I assume determines when to add air.

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2000 X5 4.4i w/ 150K miles.